No worries — the Paris Prefect of Police is on the case. He and his and his men have searched Minister D’s residence thoroughly, even using a magnifying glass to examine the tables and chairs. But they have found nothing. So they bring in one of the first famous amateur detectives: C. Auguste Dupin. He quickly finds the letter — it is hidden in plain view on a card rack in D’s room.
So, what does this old detective story have to do with modern process safety management?
Well, after a serious incident has taken place, those charged with running the investigation often find that the causes of that incident to have already been described and analyzed. The following quotation is from the book Process Risk and Reliability Management.
If you want to know the cause of an incident, look in the filing cabinet; chances are that someone described it before the event actually took place.
In other words, it is very possible that someone had already identified the potential for the incident during an audit or hazards analysis or some other evaluation program. For example, in the case of the Piper Alpha catastrophe reports had been written about the potential for a massive fire from a ruptured riser, which turned out to be a major factor in the disaster. Yet those reports were not acted upon.
"Purloined letters" on process facilities can be hidden in many places, including the following:
- Process hazards analyses
- Maintenance reports
- Inspection reports
- Prestartup safety reviews
Of course, the best time to find these letters is before an incident takes place so that management can take the appropriate corrective action.
Example: The Ethylene Oxide Reactor
The idea of information being “hidden in plain view” was illustrated during the startup of an ethylene oxide (EO) plant that had a rated production capacity of 100 tons/day. After the usual start-up hiccups, the plant was running with process conditions pretty much where they should be. In particular, the temperatures, pressures and flow rates in the EO reactor were at design values.
But the production rate of EO wasn’t 100 tons/day, it was 90 tons. Where, we asked, was the missing 10 tons?
The sketch below provides a high-level view of the process.
Ethylene and oxygen are mixed in the reactor to form an EO vapor stream that also contains unreacted ethylene and various impurities. This crude EO stream flows into an Absorber, down which flows cool water. The EO dissolves in the water; the unreacted gases leave the top of the Absorber and are returned to the Reactor. The rich water from the bottom of the Absorber is sent to a Stripper into which live steam is added. Purified EO vapor leaves the top of the Stripper, the lean water is recycled back to the Absorber.
The lean water leaving the Stripper is cooled in a Cooling Tower before it enters the Absorber. A hydrocarbon detector is placed in the air stream leaving the top of the tower.
During the start-up the hydrocarbon detector indicated that there were flammable materials in the water vapor leaving the cooling tower. The alarm was ignored. The instrumentation on the facility was not of high quality, so the alarm, in as much as anyone paid attention to it, was treated as “instrument error”.
In fact, the “instrument error” signal was an example of Poe’s purloined letter. . It was telling those responsible for running the unit the reason for the production losses. But no one recognized what they were looking at because it was hidden in plain view.
What had happened was that the temperature in the base of the Stripper was slightly too low. Hence the lean water leaving the bottom of the Stripper contained some dissolved EO. When this stream flowed through the cooling tower the EO was vaporized and was literally blown into the sky, so the hydrocarbon alarm was triggered.
Having figured out the problem the steam flow rate was increased by a small amount; the temperature at the base of the Stripper went up and — about 15 minutes later — the flow rate of EO product to the storage tanks jumped from 90 to 100 tons/day. And the cooling tower alarm stopped sending a warning signal. The failure to treat a warning signal seriously was only one of the issues to do with this event — not even the most important. Nevertheless, it is a reminder to always keep an eye out for the purloined letter that is hidden in plain view, i.e., to pay attention to all warning signs and those “must be wrong” instrument signals, no matter how unlikely or implausible they may be.